Hello there,
Are you performing the proper WSUS maintenance including but not limited to running the Server Cleanup Wizard (SCW), declining superseded updates, running the SQL Indexing script, etc.?
Have you cleaned up the superseded updates on the WSUS Server? There are many cases indicated that the issue is more related with WSUS database. I suggest to clean up the database first. Decline all the superseded updates and run the Server Cleanup Wizard to clear the superseded updates.
The below thread discusses the same issue and you can try out some troubleshooting steps from this and see if that helps you to sort the Issue.
WSUS shows Feature Updates as Not Applicable https://learn.microsoft.com/en-us/answers/questions/491233/wsus-shows-feature-updates-as-not-applicable.html
Windows 10 Clients stops reporting to WSUS Console https://learn.microsoft.com/en-us/answers/questions/317072/windows-10-clients-stops-reporting-to-wsus-console.html
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